Edge-control system for curtain coater

ABSTRACT

A coating system for a web traveling in a web-travel direction has a slot nozzle from which a curtain of a coating material falls on the traveling web and a pair of guide elements flanking the slot nozzle and having confronting and vertical inner faces engaging and defining edges of the curtain. Respective aspiration elements fixed at lower ends of the guide elements have inner faces vertically aligned with and forming vertical continuations of the inner faces of the respective guide elements, and each formed with a vertical and inwardly open aspiration groove and with a transversely extending vacuum passage open into the respective groove. Respective floors on the aspiration elements downwardly close the respective aspiration grooves and are each formed with a sharp inwardly directed edge closely spacedly juxtaposed with the web. Excess coating material is aspirated from the curtain through the vacuum passages.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US national phase of PCT applicationPCT/EP02/03776, filed 5 Apr. 2002, published 17 Oct. 2002 as WO02/081103, and claiming the priority of German patent application10117668.6 itself filed 9 Apr. 2001.

FIELD OF THE INVENTION

The invention relates to an apparatus for coating a traveling materialweb with a slot nozzle emitting a free-falling curtain of coatingmaterial and having at each side of its outlet slot a downwardlyextending guide element for an edge of the curtain, and with an elementat each edge of the web for aspirating coating material.

BACKGROUND OF THE INVENTION

In order to coat traveling webs, such as paper or cardboard webs orplastic or metal foils, so-called curtain coaters are known that applythe coating material (plastic dispersions, inks, etc.) to the web as afree-falling curtain from a slot nozzle. In order to produce a stablecurtain that is uniform over the entire coating width, WO 99/09253 (DE197 35 558 and U.S. Pat. No. 6,468,592) describes guiding the twocurtain edges each with a guide element extending from the slot nozzledown to near the web. The two guide elements are formed as bars and haveflat inner faces turned toward the curtain. So as to hold the curtain incontact with the guide elements and thus prevent the curtain edges frommoving inward, the inner face of each guide element are coated with awetting agent, for example water or the coating material, thatinfluences surface tension.

An apparatus of this type known from WO 98/47630 published 29 Oct. 1998serves for applying dispersions to a web where downstream of the slotnozzle in each edge region of the web there is an element for aspiratingthe dispersion. Suction nozzles that can be moved vertically andtransversely to the web-travel direction are used to aspirate any excesscoating on the two web edges. The excess of coating material resultsfrom the fact that the curtain is somewhat constricted at its edgeguides and thus is applied somewhat more thickly to the web in theseregions so as to produce an edge bead. This effect is known as “neckin.” The excess at the edges creates unwanted variations in thickness ofthe coated web and can lead to problems in a subsequent dryer or whenrolling up the coated web.

OBJECT OF THE INVENTION

It is an object of the invention to so improve a coating apparatus ofthe above-described type that, with simplified construction, it ispossible to achieve a uniform layer of the coating material over theentire width of the web with straight and sharp coating edges.

SUMMARY OF THE INVENTION

This object is achieved in that an aspiration element is fixed at alower end of each guide element, has an inner face that is turned towardthe curtain and aligned with an inner face of the guide element, and isformed with a throughgoing vacuum line opening into a vacuum passagethat opens into an aspiration groove that extends vertically on a lowerregion of the inner face of the aspiration element in a plane of thecurtain.

Such construction of the aspiration element ensures on the one hand atrouble-free exactly guided application of each curtain edge up toimmediately before it engages the web and on the other hand aspiratesany excess coating at the edges horizontally toward the outside and thuseliminates the so-called “neck-in” effect. A sharp and straight edge isproduced on coating the web.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross section through a curtain at an edge region of acoating apparatus;

FIG. 2 is a side view of the apparatus of FIG. 1; and

FIG. 3 is a top view on the aspirating element.

SPECIFIC DESCRIPTION

The coating apparatus shown in the drawing serves for applying adispersion adhesive (e.g. an acrylate) in an aqueous solution to asiliconized paper web. Due to its advantageous characteristics, theapparatus can also be used to apply other dispersions onto paper webs orplastic or metal foils as well as to coat paper or cardboard webs withink.

The apparatus has a slot nozzle 1 that is suspended in a frame of theapparatus above the path of the web. The slot nozzle 1 is connected to asupply of coating material and has on its underside an outlet slot 3extending the full coating width and from which the coating materialdrops as a free-falling curtain that lands on the upper surface of theweb.

At each side of the slot nozzle 1 directly against the outlet slot 3 isa bar-shaped guide element 2 that extends downward to immediatelyadjacent the web. Each guide element 2 has an inner face that is turnedtoward the curtain and that is coated with a wetting agent, for examplewith water or the coating material. To this end each guide element 2 hasabove the outlet slot 3 of the slot nozzle 1 a throughgoing hole 4 thatis connected to a feed line 5 via which the wetting agent is supplied.At its inner end the throughgoing hole 4 opens into a feed passage 14running vertically downward from above the outlet slot 3 and having anoutput end that is level with and directly adjacent the outlet slot 3.The feed passage 14 is of very simple construction, comprised of avertical straight groove formed on the outer face of the side wall ofthe slot nozzle 1 and closed when the guide element 2 is fixed to theside wall.

Preferably each guide element 2 is formed as a bar and has a thicknessof 0.1 mm to 0.3 mm and a width of about 10 to 20 mm. The length of theguide element 2 corresponds to the height of the curtain, that is thespacing between the outlet slot and the web being coated. In thisembodiment the drop height of the curtain and thus the downwardlyextending free portion of each guide element 2 is between 50 mm and 250mm.

It is significant for the invention that each guide element 2 carries atits lower end an aspirating element 6 whose inner face turned toward thecurtain is aligned with the inner face of the guide element 2. As aresult the curtain edge flowing down the guide element 2 and then ontothe aspirating element 6 is not disturbed on its way toward the web. Avacuum passage 7 leads inward through the aspirating element 6 and isconnected to a suction line 8. The vacuum passage 7 is at the lower endof the aspirating element 6 and opens into an aspiration groove 9 thatextends vertically on the lower end of the inner face of the aspiratingelement 6. The width parallel to the web-travel direction of theaspiration groove 9 is 0.5 mm to 15 mm, preferably 5 mm to 12 mm, sothat aspiration is maintained even when the curtain moves somewhat in adirection transverse to its plane, which here is parallel to the planeof the view in FIG. 2. Its depth is 0.1 mm to 2 mm, preferably 0.5 mm to1 mm. It is mounted on the lower end of the aspirating element 6 suchthat its spacing from the web is 0.1 mm to 3 mm. In order to maintainthis slight spacing, the vacuum passage 7 ends at a narrow floor 10inclined slightly toward the web and ending at a sharp edge in theaspiration groove 9. Aspiration of the thick edge of the coatingmaterial thus is effected horizontally. This has the advantage that theforces working against such aspiration are minimal and also that theedge is not smeared by the aspiration. Further horizontal orientationwithout dead zones in the vacuum passage 7 prevents aspirated coatingmaterial from forming deposits.

In order to prevent depositions from forming in the aspiration groove 9and in the vacuum passage 7, a rinse liquid, for example water, isflushed through them. To this end, two rinse passages 11 extend parallelto the suction line 8 and open into the aspiration groove 9 at a slightspacing upstream and downstream relative to the web-travel direction ofthe vacuum passage 7. The rinse liquid is carried off with the aspiratedcoating material by the suction line 8. The suction line 8 is to thisend connected to the intake of a pump.

To be able to do some adjustment of the position of the edge of thecurtain transverse to the web-travel direction, both guide elements 2are formed of a limitedly flexible and elastically bendable material,for example spring steel. A support beam 12 extends downward from theside wall of the slot nozzle 1 and is closely juxtaposed with each guideelement 2 in order to brace and position it. The support beam 12 is madeof a rigid material, for example of fairly thick steel, and ends justshort of the aspirating element 6. At its lower end the support beamholds an externally operable adjustment element, for example a screw 13,that can press from outside against the guide element 2. The adjustmentscrew 13 can limitedly move the free end of the guide element 2 with theaspirating element 6 carried on it transversely of the web-traveldirection in order to exactly set the position of the aspiration element9. In addition the adjustment elements 13 also allow the shapes of theguide elements 2 to be adjusted within limits in order to optimize theguiding and maintenance of the curtain edges.

1. A coating system for a web traveling in a web-travel direction, the coating system comprising: a slot nozzle from which a curtain of a coating material falls on the traveling web; a pair of guide elements flanking the slot nozzle and having confronting and vertical inner faces engaging and defining edges of the curtain; respective aspiration elements fixed at lower ends of the guide elements, having inner faces vertically aligned with and forming vertical continuations of the inner faces of the respective guide elements, and each formed with a vertical and inwardly open aspiration groove and with a transversely extending vacuum passage open into the respective groove; respective floors on the aspiration elements downwardly closing the respective aspiration grooves and each formed with a sharp inwardly directed edge closely spacedly juxtaposed with the web; and means for aspirating excess coating material from the curtain through the vacuum passages.
 2. The coating system defined in claim 1 wherein the aspiration groove ends at the sharp edge 0.1 mm to 3 mm from the web.
 3. The coating system defined in claim 2 wherein the aspiration groove has a horizontal width parallel to the web-travel direction of 0.5 mm to 15 mm.
 4. The coating system defined in claim 2 wherein the aspiration groove has a horizontal depth perpendicular to the web-travel direction of 0.1 mm to 2 mm.
 5. The coating system defined in claim 1 wherein each guide element is formed above the respective aspiration element with an inwardly open rinse passage, the system further comprising means for feeding a rinse liquid to the rinse passages.
 6. The coating system defined in claim 1 wherein each floor is inclined at a slight acute angle inward and downward toward the web.
 7. The coating system defined in claim 1 wherein each guide element is vertically elongated and is elastically bendable in a plane of the curtain.
 8. The coating system defined in claim 7, further comprising respective edge-adjustment means bearing radially inward on the guide elements for elastically inwardly deforming same and thereby establishing a curtain width. 